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Constructed Wetlands in theNorthern Territory

Guidelines to Prevent MosquitoBreeding

Medical EntomologyCentre for Disease Control

NT Department of Health and FamiliesMay 2008

For further information contact;

Medical Entomology Branch

Centre for Disease ControlDepartment of Health and Families

PO Box 40596

Casuarina NT 0811

Ph: 08 89228902

Fax: 08 89228820

Email: medicalentomologyrdh@nt.gov.au

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Constructed Wetlands for Water Sensitive Urban Design in the NT

Guidelines to Prevent Mosquito Breeding

1.0IntroductionConstructed Wetlands are designed to manage and treat urban stormwater, and can have

various forms such as a simple open lake system, or periodically flooded shallow vegetated

basins. Constructed wetlands have the potential to be much more productive breeding sites for

mosquitoes compared to natural wetlands due to high nutrient from urban runoff, therefore

constructed wetlands in the Northern Territory need to be appropriately designed and

managed to prevent mosquito breeding.

Mosquito species that are likely to breed in freshwater constructed wetlands in the Top End of

the NT include the major arbovirus vector Culex annulirostris (the common banded

mosquito), various Anopheles species (potential malaria vectors) and the pest mosquitoes

Coquillettidia xanthogaster (the golden mosquito) and Mansonia uniformis (the water lily

mosquito). Constructed wetlands in tidal areas could become breeding sites for the northern

salt marsh mosquitoAedes vigilax, the saltwater Culex species Culex sitiens, and the saltwater

and brackish waterAnopheles speciesAnopheles farauti s.s. (An. farauti no.1) andAnopheles

hilli.

There may be conflicts between the design for water treatment alone and the design features

to prevent mosquito breeding. However it is important to consider the potential for mosquito

breeding while designing and constructing wetlands, as both aims can often be accommodated

in a compromise design. Constructed water bodies that become mosquito breeding sites will

not only affect nearby residents by increasing pest and potential mosquito borne diseasetransmission, but would incur a significant cost by the managing authority (usually a local

council) for associated mosquito survey/control and site rectification.

The purpose of this guideline is to assist developers and land managers in deciding on an

appropriate wetland design and management regime, which meets the public health

requirements as well as water treatment and other requirements of a constructed wetland.

2.0Mosquito species and constructed wetlands

The common banded mosquito Culex annulirostris

This species breeds in the vegetated margins and pools in permanent and semi-permanent

freshwater swamps, creeks and floodways, temporary flooded vegetated ground pools, and inhigh nutrient water such as effluent discharge and urban stormwater drains. Potential breeding

sites in constructed wetlands would include any shallow area containing semi-aquatic or

aquatic vegetation, as well as vegetated stormwater drains and channels, areas of high nutrient

water pools (ie stormwater pipe outfalls), and flooded vegetated depressions in landscaped

areas.

Culex annulirostrisis most common within 2km of productive breeding sites, but can disperse

up to 15km from major breeding sites. Culex annulirostris is the most important vector of

arboviruses in the NT. It is recognised as a good vector of Murray Valley encephalitis virus

(MVEV), Kunjin virus (KUNV), Ross River virus (RRV) and Barmah Forest virus (BFV).

Many other arboviruses have been isolated from this species.

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Anopheles mosquitoes

Anopheles mosquitoes generally breed in fresh and brackish water swamps or creeks.

Potential breeding sites in constructed wetlands include shallow flooded areas containing

semi-aquatic vegetation and vegetated stormwater drains and channels.Anopheles mosquitoes

are usually most common within 1.6km of breeding sites, although some species such as

Anopheles hilli will fly several kilometres from their breeding sites. Some Anophelesmosquitoes such asAn. farauti s.l. andAn. annulipes s.l. are regarded as potential vectors of

malaria.

The frosty mosquito Culex gelidus

This species breeds in freshwater ground pools, swamps and containers. In the NT, this

species has often been associated with high organic wastewater ponds in piggeries, abattoirs,

dairies and sewage treatment facilities. Potential breeding sites in constructed wetlands would

be areas of high nutrient ponding, such as at stormwater pipe discharge sites. This species is a

potential vector of Japanese encephalitis virus, MVEV, KUNV, RRV, BFV and other

arboviruses.

The brown house mosquito Culex quinquefasciatus

This species breeds in septic tanks, polluted stormwater drains, effluent treatment facilities,

flooded depressions with high organic content and water filled domestic receptacles. Potential

breeding sites in constructed wetlands include high nutrient ponding areas at stormwater pipe

outfalls. Culex quinquefasciatusis usually most common within 500m of productive breeding

sites. Culex quinquefasciatus is only a pest mosquito in Australia.

The golden mosquito Coquillettidia xanthogaster

This species breeds in swamps, billabongs and creeks with semi-aquatic and aquatic

vegetation, particularly semi-aquatic reeds. Potential breeding sites in constructed wetlands

would include any shallow flooded areas with semi-aquatic vegetation. Coquillettidia

xanthogaster is usually most common within 3km of productive breeding sites, and is not

regarded as a human disease vector in Australia.

The water lily mosquitoMansonia uniformis

This species breeds in similar habitats as Cq. xanthogaster, but are more associated with

floating vegetation. Potential breeding sites in constructed wetlands would include any

shallow flooded areas with semi-aquatic and aquatic vegetation. This species is most commonwithin 2km of breeding sites, and is not regarded as a human disease vector in Australia.

The northern salt marsh mosquitoAedes vigilaxNatural breeding sites forAe. vigilax are temporary flooded areas in tidal to brackish swamps,

creeks, salt marshes, upper mangrove areas and coastal dune depressions. Constructed

wetlands could create breeding sites for this species if they are built in or adjacent to tidal

areas. Aedes vigilax will breed in depressions within salt influenced wetland systems that

periodically dry and then become inundated with tide water, stormwater or rain. This species

will also breed in inappropriately landscaped areas surrounding tidal wetlands, and in

stormwater drains with tidal influence.Aedes vigilax is a major pest mosquito.

Aedes vigilaxis most common within 5km of breeding sites, but can fly up to 50km in pest

numbers from large breeding sites.Aedes vigilax is a vector of RRV and BFV.

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The saltwater Culex mosquito Culex sitiens

Breeding sites for Cx. sitiens are the same as forAe. vigilax, although Cx. sitiens only breeds

when extended saline ponding occurs. Culex sitiens is most common within 2km of breeding

sites. Culex sitiens is regarded as a potential vector of RRV disease, and can be an appreciable

pest near productive breeding sites.

3.0Constructed wetlands and the potential for mosquito breeding

Constructed wetlands can be either relatively simple lake systems, or more complex systems

that include shallow areas of flooded semi-aquatic vegetation. It is the shallow vegetated areas

of these wetlands that have the greatest potential for mosquito breeding, with their potential

rising dramatically as organic loads increase.

Simple freshwater lake systems that are constructed with deep water and relatively steep sides

have been built in areas of Darwin and Palmerston, and the lakes themselves have not become

significant mosquito breeding sites. The lakes generally have minimal or a thin margin of

semi-aquatic reed growth, as well as relatively steep sides and deep water to minimise theextensive colonisation of semi-aquatic vegetation and facilitate fish survival. The potential for

mosquito breeding in such lake systems usually only arises if regular maintenance is not

conducted to remove silt deposition in inlet areas and excess semi-aquatic reed growth, or

when fish populations are eliminated.

Shallow vegetated wetlands provide a favoured habitat for mosquito larvae. The potential for

productive mosquito breeding in shallow vegetated wetlands is dependant on the extent and

density of semi-aquatic reed growth. Dense shallow mats of fallen reeds in a constructed

wetland will give rise to productive mosquito breeding. These dense shallow mats of reeds

would provide mosquito habitat both during initial flooding in the early wet season, and

during the late wet and early to mid dry season when reeds fall over as water levels recede.

Constructed wetlands with extensive shallow areas of semi-aquatic vegetation will require a

high degree of maintenance and will be costly to minimise mosquito breeding. Shallow

wetlands receiving stormwater flow are also likely to be more productive mosquito breeding

sites than comparable natural wetlands, due to the higher nutrient input from stormwater

discharge.

Constructed wetlands are also likely to attract animals, which may act as reservoirs of various

arboviruses, for example water birds which are hosts for the potentially fatal Murray Valley

encephalitis virus, and marcopods (wallabies), which are hosts for Ross River virus. It is

therefore important not to have the combination of animal reservoirs and the mosquito vectorsof disease, particularly within mosquito flight range of residential areas.

4.0Risk assessment

There should be a risk assessment conducted to determine the potential for mosquito breeding

in constructed wetlands. For example, a shallow constructed wetland within 2km of urban

residential areas will pose a high risk of creating pest and arbovirus transmission problems.

Mitigation measures to reduce mosquito breeding in such a wetland would be ongoing and

costly, and are discussed further in this document. Conversely, a deep, steep sided lake would

pose minimal mosquito breeding issues for adjacent residents.

When there are likely to be significant mosquito breeding issues with a particular wetlanddesign, consideration should be given to an alternative design with a lower mosquito breeding

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potential. An alternative design may reduce the water treatment efficiency of the wetland to

some extent, but in most instances there should be a balance between water treatment and

public health. Both designs should be compared for positives and negatives from all

perspectives (eg Water treatment efficacy, mosquito breeding, public safety, maintenance

costs etc), before the final design is chosen. It should be noted that it would be a requirement

under the Public Health (General Sanitation, Mosquito Prevention, Rat Exclusion andPrevention) Regulations to prevent mosquito breeding

For each constructed wetland, the time for significant levels of contaminants to settle out of

the water would need to be determined, with design and management measures tailored to suit

these calculations. For example in Brisbane, it is suggested 72 hours is a preferable period of

detention in the macrophyte (vegetation) zone of a constructed wetland to allow removal of

contaminants. Therefore there in some situations there may be no need to retain water for

periods greater than 3 days in a heavily vegetated shallow area. This would generally avoid

mosquito breeding, as mosquito larvae take from 6 to 10 days to complete their aquatic

development stages.

5.0Design considerations

5.1 Siting

Preferably, constructed wetlands should be sited in an open area exposed to wind, to

maximise the impact of wave action to disrupt mosquito breeding. To maximise the effect of

wind, the water body should be orientated so its long axis is in line with known prevailing

wind direction (south-east dry season winds and north-west wet season monsoon winds for

Darwin region).

If practical, constructed shallow vegetated wetlands likely to breed mosquitoes should be

sited at least 1.6km from any urban residential areas. This would provide a buffer distance tominimise the potential for mosquito breeding to impact on residents. Constructed wetlands in

tidal areas should generally be avoided due to the inherent difficulties in constructing and

maintaining a wetland in a tidal area. In tidal areas, the inundated areas would generally need

to be free draining on a daily basis, or have steep sides around a salt to brackish water lake.

Wetlands should be sited in an area where a relatively simple design can be achieved, as

wetlands with simple shapes and a low edge to area ratio have a lower potential to become

productive mosquito breeding sites.

5.2 Hydrology

There should be a component of a constructed wetland that permanently retains water

throughout the year. For example, a lake should retain water at one end to provide a refuge for

fish during the dry season. A constructed wetland that completely dries and then re-floods

will lack mosquito predators for a short period after re-flooding, and could become a short

term mosquito breeding site. A constructed wetland should however be allowed to recede

during the dry season to some extent, to allow maintenance of edges (eg silt and vegetation

removal). Lakes or water features in the Northern Territory, with virtually no rain for 5-6

months, need to be designed to retain water during these long periods of no rain, or be

periodically topped up with water.

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5.3 Aquatic vegetation

The provision of semi-aquatic and aquatic vegetation is generally necessary to remove

nutrients from water, and is required to reduce the potential for algal blooms. Relatively

sparse or narrow marginal areas of emergent vegetation are unlikely to lead to mosquito

breeding, as fish access would not be restricted. Potential problems will arise if regular

maintenance is not conducted and semi-aquatic/aquatic vegetation becomes dense andextensive and creates harbourage for mosquito larvae.

Semi-aquatic reeds such as Typha sp. and, Eleocharis sp. can provide major habitat for

mosquito larvae, but are important for removing nutrients from the water. Semi-aquatic reeds

can be restricted to small areas of upright vegetation by the use of sub-surface concrete boxes

or barriers to prevent rhizome spread. This design will allow predator access and prevent

mosquito breeding, while at the same time provide refugia for fish. Semi-aquatic reeds in

shallow lakes or basins can quickly spread and create extensive areas of vegetation which will

enhance mosquito breeding. If the design of a wetland incorporates flooded semi-aquatic

vegetation, there would be a requirement for at least an annual maintenance program to

harvest vegetation.

Semi-aquatic reeds should not be allowed to become dense as pictured above (Photo A-

Coonjimba Billabong in Jabiru), as this will give rise to high levels of mosquito breeding and

ground control with mosquito larvicides in the dense vegetation would be very difficult.

Alternatively, semi-aquatic reeds can be planted along a thin margin at the upper water limit

of a steep margin of the wetland. The water level can be seasonally manipulated during the

dry season and stranded vegetation can be easily maintained or removed (see Photo B). The

spread of rhizomes of semi-aquatic reeds such asEleocharis or Typha could be limited by

constructing a narrow concrete retaining wall along the wetland margin. Generally if steep

sides and deep water (at least 1.5m deep) are provided, the spread of reeds would be restricted

to the shallow upper margin.

The same principles also apply to other semi-aquatic plants such as sedges, with sparse

vegetation unlikely to lead to mosquito breeding. Aquatic plants such as water lilies are

recommended for smaller shallow wetlands, as they provide shade for fish.

Annual maintenance is generally required to remove dead semi-aquatic vegetation, either by

physical removal or by burning. Semi-aquatic vegetation that has begun to spread beyondtheir desired location should also be physically removed, or controlled by herbicide.

5.4 Water quality

Water quality in constructed lakes should be maximised by utilising some form of mechanical

aeration, which can be achieved by using fountains or waterfall features. Well circulated,

oxygenated water bodies are less likely to produce algal blooms and are less likely to produce

fish death. The use of fountains in smaller water bodies is also useful to disturb the water

surface and disrupt mosquito breeding. Smaller fountains located near the margins of a lake

could be utilised to create disturbance to the shallow edges where mosquito breeding usually

occurs. Mechanical aeration would be particularly important during the late dry season, when

temperatures are high and oxygen levels are likely to be low.

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5.5 Lake systems

Lake systems should be simple in design, and generally should have steep sides (at least

1V:2H) and relatively deep (1.5 to 2m) wet season stabilised water level. There can be areasof semi-aquatic vegetation and aquatic vegetation provided to treat water, although vegetation

should be limited to relatively small stands that are regularly maintained by

harvesting/physical removal, or extensive vegetated areas that are only flooded for 2-4 days.

Stormwater should flow into the deepest section of any lake.

There may be issues with public safety when providing steep edges. In those instances when

shallow edges are required, a concrete vertical lip (200-300mm) should be provided at the

lake margin to maximise the effect of wave action. The shallow area of the lake adjacent to

the vertical lip would need to be maintained free of semi-aquatic vegetation. The concrete lip

can be aesthetically acceptable if constructed appropriately (see Photo D).

5.6Constructed wetlands with shallow vegetation treatment zonesThese types of wetlands include a shallow vegetated component (marcophyte zone) to treat

stormwater, and a deep lake or deep pools to provide refuge for fish. These wetlands require

careful design considerations, as the shallow vegetation treatment zones could become

productive mosquito breeding sites.

Dry season flows need to be directed into the deepest section of the permanent lake or deep

pool within the wetland system. This deeper section is where fish and other aquatic predators

are likely to be present, and where the potential for semi-aquatic vegetation growth is

minimal. Silt traps would be required where stormwater drains lead into the constructed

wetland.

5.6.1 Wetlands with detention vegetation zones

Wetlands that have vegetated components which detains water for a period of 48-72 hours,

with water then draining into a lake or deep pools are unlikely to become significant mosquito

breeding sites. Detention in an extensively vegetated area for 48-72 hours would not breed

mosquitoes, as this period of time would not allow full larval development. This detention

period should still provide removal of fine sediment and soluble pollutants. This type of

design is appropriate for any proposed shallow constructed wetlands within 2km of residentialareas in the Northern Territory.

There should be periodic inspections during the wet season to ensure the detention zone does

not pond water for greater than 3-4 days. Any shallow depressions that pond water for greater

than 4 days should be earmarked and rectified during the following dry season. Annual

maintenance would be required to remove dead vegetation, and harvest or remove vegetation

that has become dense or spread to other areas of the wetland.

5.6.2 Wetlands with retention vegetation zones

This design includes a shallow vegetation area that ponds water for extended periods, with a

lake/deep pools provided for fish refuge during the dry season. Wetlands with shallow heavilyvegetated treatment zones, which retain water for the duration of the wet season and into the

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early to mid dry season will breed mosquitoes. This type of design should typically be

avoided unless detailed studies indicate these systems can be designed to remove sediment,

pollutants and nutrients and not breed mosquitoes.

Wetlands with vegetated retention zones will require a comprehensive monitoring and

maintenance program to minimise mosquito breeding. This includes weekly adult mosquitomonitoring around the wetlands, as well as monitoring in nearby areas, to establish if

mosquito populations are originating from the constructed wetland. Weekly larval surveys

during the wet season to mid dry season would also be required to locate any actual mosquito

breeding within the shallow vegetated component of the wetlands. Annual maintenance such

as vegetation removal and silt removal would also be required, as well as regular visual

inspections to ensure there are suitable fish populations. This monitoring and maintenance

program would need to be conducted by the landholder or responsible authority.

Wetlands designed with vegetated retention zones need to have an emergency drainage

provision provided, which will allow the shallow component to be drained over a period of a

few days. The emergency drainage would be utilised if significant mosquito breeding islocated and mosquito larval control operations are unlikely to be effective (ie if ground

control is impossible due to dense vegetation, lack of required personnel). This could be

achieved by installing a pipe system or contour system in the lowest point of the shallow area,

to direct water to the lower lake/deep pool component. A gate in the bund wall separating the

shallow vegetated retention zone from the deep lake, with a provision to close and open when

needed, could an option for emergency drainage (ie similar to a lock system for a marina).

The shallow area would require annual maintenance to ensure there are no isolated

depressions that could pond water for extended periods after it has been drained, and to ensure

the emergency drainage system is in working order.

5.7 Constructed wetlands in or adjacent to tidal areas

Constructed wetlands in or adjacent tidal areas have the great capacity to breed mosquitoes.

This is due to saline mosquito species such as Aedes vigilax and Culex sitiens being able to

breed in high numbers in vegetation free shallow water areas.

It is very difficult to achieve a depression free surface in tidal areas, particularly in shallow

extreme upper tidal areas that are infrequently inundated by tides. These areas tend to have

minimal slope, and subtle changes in vegetation growth or silt deposition can create shallow

depressions conducive to mosquito breeding. A relatively deep (1-2m), steep sided tidally

influenced lake with a tidal water retaining barrier is one design that can be recommended for

water retention in tidally affected areas. This design has worked very well at Vesteys Beach inDarwin (see Photo E).

5.8 Silt traps

Silt traps are required to capture coarse sediments and minimise silt deposition in wetlands.

Silt traps are best designed with a hard floor (eg concrete) and steep sides (preferably

concrete), with a suitable access ramp for machinery. Silt traps need to be positioned at

stormwater discharge points. Due to the likelihood of dry season flows, there should be

provisions provided in all silt traps to dam or divert dry season flows to a deep section of thereceiving lake or water body, so annual maintenance can be performed. Silt should be

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removed from the silt trap on at least an annual basis by the responsible authority. Silt traps

with vegetation should be designed to completely drain within 3-4 days, alternatively silt traps

should incorporate a design that does not include vegetation.

5.9 Stormwater drains

All urban stormwater drains leading into constructed wetlands must be the standard

underground stormwater pipe or concrete invert open drain, to prevent the creation of

mosquito breeding within stormwater drains. This includes ensuring all stormwater road side

entry pits, grate inlet pits and letterbox pits are free draining, and ensuring grassed swale

drains have concrete low flow inverts when there is the likelihood of low flows during the dry

season. Dry season low flows must be directed into the deepest section of any water body.

5.10 Landscaping

Appropriate landscaping of areas surrounding constructed wetlands is vital, as poorly draining

surrounds has caused many mosquito problems around constructed wetlands in Darwin.Appropriate grades would need to be applied to all landscaped areas surrounding wetlands, to

allow the sheetflow of water into the wetland. More extensive and wider surrounds may need

swale drains with concrete inverts leading into wetlands.

5.11 End point of discharged water

There must be no dry season discharge of water from a constructed wetland, unless the

discharge is directed to a lake or the sea via an appropriate drain that will not give rise to

mosquito breeding. Constructed wetlands without an appropriate end point for dry season

discharge water should have sufficient capacity to retain water, or be appropriately managed

to prevent the dry season discharge of water.

Wet season overflows from constructed wetlands should be directed to the 4.0m AHD level

for those developments adjacent to tidal areas, or to a defined river or free draining creekline

for those developments away from tidal areas. The overflow should be suitably designed and

maintained such that it will not have the capacity to breed mosquitoes.

6.0Maintenance

Before the construction of any lake system or water sensitive urban design strategy, the

relevant authority that will assume control of the water feature would need to be determined.

The relevant authority would then need to develop a mosquito management plan in

consultation with the developer and Medical Entomology of DHCS, to ensure the water

feature is managed appropriately by the relevant authority to avoid any appreciable mosquito

breeding.

Management measures would involve annual maintenance such as; removing silt from

sediment traps; removing silt from lakes/deep pools; harvesting semi-aquatic vegetation from

shallow treatment zones and lakes/deep pools; burning/removal of dead vegetation from theshallow treatment zone; rectification of surface depressions in shallow treatment zones;

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desilting and removal of vegetation from open drains and filling and grading landscaped areas

to remove surface depressions.

7.0Mosquito monitoring

Any wetland with a retention vegetation zone should have a mosquito monitoring program

established for the life of the wetland. Mosquito monitoring should involve adult trappingonce a week at the wetland, and at a site at least 500m away from the wetland, to allow an

evaluation of the origin of mosquito numbers at the wetland site. Recommended adult

mosquito traps would be carbon dioxide baited Encephalitis Virus Surveillance (EVS) traps,

as they are the same traps used in most other parts of the NT. Indicator mosquito species in

the adult mosquito traps would be the species with a low effective flight range (eg Anopheles

species,Mansonia uniformis, and Culex quinquefasciatus, and also Culex annulirostris).

Larval mosquito surveys are required on a weekly basis during the wet season and early to

mid dry season for any shallow flooded vegetated retention areas. Larval surveys involve

surveying in shallow areas with dense vegetation growth with a 300ml ladle. A larval

density* of 1 larvae per 2 ladle dips or higher is likely to indicate a potential mosquitoproblem for adjacent residents. If the larvae are identified as important pest or disease

mosquitoes and if the breeding area is large, some form of mosquito control would be

required, such as draining the retention area or using a suitable mosquito larvicide. An

evaluation of the cause of the breeding should then be conducted, with rectification measures

implemented to prevent further breeding.

*Please note that these larval densities are only suggested threshold levels. If complaints from residents or adult trappingreveals significant pest mosquito problems despite larval counts being lower than the threshold, then the threshold will need

to be re-evaluated.

8.0 Mosquito controlMosquito control capabilities is vital for constructed wetlands. In general biological control

agents such as fish are the most efficient method of controlling mosquito breeding, providing

fish have sufficient access to all areas of the wetland. Fish species to be used in constructed

wetlands must be sourced from local waterways, to prevent the introduction of exotic fish

species. In general the rainbow fish (Melanotaenia spp.) are very hardy and should be stocked

in all wetlands. Other fish species that can be stocked in constructed wetlands include blue

eyes (Pseudomugil spp), glass perchlets (Ambassis spp.), grunters (Leiopotherapon spp.) and

gudgeons (Mogurnda spp.).

Along with fish, annual vegetation maintenance should keep mosquito breeding to minimal

levels. However, there may be periods of high mosquito breeding that would requiretemporary insecticide control until the cause of the breeding is rectified. The use of

insecticides can quickly control mosquito breeding, but is not recommended as a long term

strategy due to potential issues with insecticide resistance, long term cost of maintaining a

mosquito control program and the possibility of the insecticide applications not being able to

target all areas of breeding. Mosquito breeding can be controlled with the specific and

ecological friendly insecticides Bacillus thuringiensis var. israelensis, Bacillus sphaericus or

methoprene until a solution to prevent/minimise mosquito breeding is implemented.

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9.0 Summary

Constructed wetlands can potentially create habitat for mosquito larvae. There are however

specific design and management options that can be used to minimise or prevent mosquito

breeding. The general wetland design and management requirements by the responsible

authority are listed in order of priority. This information is also displayed in Appendix 1.

1. Wetlands constructed as a deep (1-2m wet season stabilised water level), steep sided (at

least 450angle or 1V:2H) lake, with stormwater discharged to the deepest point via a silt trap.

Management requirements include annual removal of silt and semi-aquatic vegetation.

2. Wetlands with detention vegetation treatment zones. The wetland should include a deep,

steep sided lake/deep pools with dry season flows directed to the deepest point, while the

vegetation treatment zone should only pond water for a period of 3-4 days. Management

requirements include annual inspections of the lake margins and removal of silt and marginal

vegetation, and annual inspections of the vegetation treatment zone, with maintenance

conducted to remove silt, isolated depressions and dead or lodged vegetation.

3. Wetlands with retention vegetation treatment zone. There should be a main lake/deep pools

provided with deep and steep sides. Dry season water should discharge into the deepest point

of the lake. There should be an emergency provision provided in the retention zone to allow

rapid drainage into a lake/deep pool if mosquito breeding becomes a problem. A weekly adult

mosquito monitoring and larval mosquito monitoring program is required to ensure mosquito

populations do not reach pest or public health risk levels. Annual maintenance is required to

remove vegetation from the retention zone, remove silt and rectify isolated depressions in the

retention zone, and remove semi-aquatic vegetation and silt from the main lake.

In conclusion, constructed wetlands have the potential to create new mosquito breeding sites

that could impact on the public health of nearby residents. Design aspects and management

options should be carefully considered before construction commences. Each wetland will

require a case by case analysis using these guidelines, which have been developed to assist

developers and land managers in choosing a suitable design.

Prepared by Allan Warchot and Peter Whelan

Medical Entomology

May 2008

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10.0 References and further reading

Brisbane City Council Water Sensitive Urban Design Practice Note Series. Practice Note 6

Constructed Wetlands.

Chironomid midge and mosquito risk assessment guide for constructed water bodies, MidgeResearch Group of WA August 2007.

Department of Medical Entomology, University of Sydney. Freshwater Wetlands (natural and

constructed). Mosquito production and management.

Guidelines for preventing biting insect problems for urban residential developments or

subdivisions in the Northern Territory, Medical Entomology, Department of Health and

Community Services 1997.

Larson, H.K. & Martin, K.C. (1989), Freshwater fishes of the Northern Territory, Northern

Territory Museum of Arts and Sciences.

Merianos, A.,Farland, A. M.,Patel, M.,Currie, B.,Whelan, P. I.,Dentith, H. &

Smith, D. (1992), A concurrent outbreak of Barmah Forest and Ross River virus disease in

Nhulunbuy,Northern Territory, Comm Dis Intel, vol. 16, no. 6, pp. 110-111.

Russell, R. C. (1999), Constructed wetlands and mosquitoes: Health hazards and

management optionsAn Australian perspective,Ecological Engineering 12 (1999) 107

124.

Tai, K. S.,Whelan, P. I.,Patel, M. S. & Currie, B. (1993), An outbreak of epidemic

polyarthritis (Ross River virus disease) in the Northern Territory during the 1990-1991 wetseason. Medical Journal of Australia,vol. 158, pp. 522-525.

Whelan, P.I. (1997), Problem mosquito species in the Top End of the NT Pest and vector

status, habitat and breeding sites, Medical Entomology Branch, Department of Health and

Community Services.

Whelan, P. I.,Merianos, A.,Patel, M.,Tai, K. S. & Currie, B. (1993), 'The epidemiology of

arbovirus infection in the Northern Territory 1980-92', Arbovirus Research in Australia,vol.

6,Proc. of the 6th Arbovirus Conference, 1993.

Whelan, P. I. & Weir, R. P. (1993), 'The isolation of alpha and flavi viruses from mosquitoes

in the Northern Territory 1982-1992',Arbovirus Research in Australia, vol. 6,Proceedings of

the 6th Arbovirus Conference, 1992.

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Appendix 1